Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, incorporated herein by reference.
Significant advances have been made in manufacturing of LEDs emitting white light. Currently, white light LEDs are commercially available which generate more than 100 lumens per watt. This is comparable to the performance of fluorescent and HID lamps. In addition, these LEDs offer other advantages such as longer operating life, shock/vibration resistance and design flexibility because of their small size. As a result, white light LEDs are gaining acceptance as a replacement for traditional incandescent sources, compact fluorescent and HID for illumination applications such as signage, accenting, and pathway lighting, downlighting, parking lot and roadway lighting. The white LEDs can be used alone or in conjunction with colored LEDs for a particular effect.
The electrical characteristics of LEDs are such that small changes in the voltage applied to the LED lamp will cause appreciable current changes. In addition, ambient temperature changes will also result in LED current changes by changing the forward drop across the LEDs. Furthermore, the lumen output of LEDs depends on the LED current. The existing electrical power supplies for LED light sources are designed to precisely regulate the LED current to prevent luminous intensity variations due to input AC voltage variations and ambient temperature. Operation of LED lamps at excessive forward current for a long period can cause unacceptable luminous intensity variations and even catastrophic failure. In addition, current electrical power supplies do not minimize power consumption to maximize energy savings.
It is often desirable to provide a dimming capability to LEDs and lighting fixtures employing LEDs. Known ways of dimming LEDs include pulse-width modulation (PWM) “chopping” of the current waveform and analog reduction of the amplitude of the current waveform. Unfortunately, using known analog amplitude reduction and PWM dimming it is difficult to obtain good efficiency and good performance over an entire dimming range of 0% light output (no light output) to 100% light output(full light output). Many known high efficiency LED drivers use a switch mode converter to regulate the current to the LED's. To achieve “deep dimming”, (e.g., dimming to less than 5% and up to 30%), PWM pulsing of the LED current is typically used to ensure proper operation of the LED's. With a current source output, PWM dimming requires a shunt switch that shunts the LED current during the “off” pulses of the PWM cycle. As such, relatively high losses are realized in the main converter and the shunt switch because the current to the LEDs is at a comparatively high level, even though only a portion is of the current is being delivered. Accordingly, known shunt switches and their methods of use are comparatively inefficient in LED applications involving dimming. In addition, the efficiency (Im/W) of LED's us comparatively high at lower drive currents, and as a result known PWM dimming methods are less efficient than known analog dimming methods. However, analog dimming also has some disadvantages at low dim levels. For example, if the LED current is less than approximately 5% and as great as 30% of the full output rating, light levels might not be uniform between different LEDs, color shifts can occur, and at very low current levels efficiencies of the LED's are also comparatively poor. In addition, the driver electronics become more difficult as the current levels drop below 1%, offset voltages and electrical noise in the current sensing circuitry become a major concern. At dim levels below 0.1% these issues make analog dimming undesirable.
Thus, there is a need in the art to provide dimming of LEDs that overcomes at least the drawbacks of known dimming methods described above.